The present invention relates to a delivery system for conditioning and conveying molten thermoplastic material produced in a melting furnace. The delivery system receives the thermoplastic material from the furnace and discharges at some desired temperature and consistency for forming a product. The invention more particularly pertains to a glass delivery system wherein the glass is conveyed through a conduit and simultaneously conditioned to a desired relatively uniform temperature and viscosity by means of a heat exchange device for efficiently and economically removing heat from the glass as it passes therethrough. The conduit may be immersed or surrounded by stagnant glass. Heaters, adapted to operate in molten glass are located about the conduit, provide uniform heat input into the glass within the conduit and prevent localized hot spots.
Vertically oriented electric glass melting furnaces have been known in the prior art for some time, but it has been only in the last several years that such furnaces have been brought to large scale commercial application. In more recently developed furnaces, such as the type illustrated in U.S. Pat. Nos. 2,993,079, 3,524,206, 3,583,861, 3,725,558, 3,742,111, 3,942,968, 4,029,887 and 4,143,232, glass forming batch materials are fed to the upper end of a vertical chamber and refined molten glass is withdrawn from the bottom of the chamber. High quality glass is thus produced in a single vertical chamber, with melting occurring in an upper portion thereof and preferably some refining occurring at the bottom portion.
The molten glass withdrawn from the electric furnace is usually received within a laterally extending connected channel situated at one side of the furnace bottom and thereafter directed through a vertical passageway or riser to a mixing chamber and/or a forehearth. For example in U.S. Pat. No. 3,942,968 to Pieper, the molten glass is withdrawn laterally from the furnace through a connected channel, thereafter is directed to a riser portion where colored materials may be added, from the riser to a downwardly extending mixing chamber, laterally from the chamber through a second connected channel, then upwardly through a second riser, and finally to a forehearth or feeder. In the Pieper system, the delivery passageway extending from the furnace bottom is formed in refractory block material of the contiguous walls of the furnace and riser, and an electrode is positioned in the passageway. It will be appreciated by those skilled in the art of melting glass that the passageway electrode may not be used when coalesced forming or batch material surrounds the electrode because the cold material will not be electrically conductive.
In some electric glass melting furnaces heretofore employed, a refractory metal delivery conduit extends from near the center of the bottom of the furnace to the confines or passageway of the connected channel. The conduit is either placed on the furnace bottom wall or is laid into a trough incorporated in the furnace bottom wall, and the conduit is protected from exposure to solid or liquid contaminants, which originate in the batch and sink through the molten glass, by a cover or refractory block. The delivery system of such type of prior art furnace is provided with devices for heating the cold glass or raw material initially within the conduit because, during the startup or beginning stage of operation of the furnace, the heat conducted through the conduit from the molten glass in the furnace and connected channel to the glass or raw materials is not sufficient to melt all the cold batch material initially within the conduit.
In the U.S. Pat. No. 4,029,887 to Spremulli, an apparatus is disclosed for heating glass or raw materials within a delivery conduit extending from an electric glass melting furnace to a connected channel. The conduit is made of an electrically conductive refractory metal material such as molybdenum and is used to conduct current from inside the furnace to its exit end in the channel. Joule effect heating between the entry and exit ends of the conduit and electrodes in the furnace and connected channel indirectly cause the cold glass or raw materials within the conduit to partially melt, to the extent that the materials within the conduit begin to flow therefrom. A flange assembly for use with the molybdenum conduit is also disclosed. In the Spremulli patent, the delivery conduit is connected to the furnace with a forehearth channel wherein the glass is conditioned for delivery to an outlet end thereof and a forming means.
In the British Pat. No. 1,412,599 commonly assigned to the assignee herein, a delivery system utilizing stationary mixing devices and a heat exchange vessel is disclosed. The system does not consider the problem of high heat loss since it is located downstream of the forehearth in a forming operation.
In a series of related U.S. patent applications, Ser. Nos. 243,811, 244,001 and 244,024 filed this same date and assigned to the assignee herein, various arrangements of glass melting and transport systems are disclosed in detail. It should be understood that although significant differences exist with such disclosures and the present invention, to the extent necessary, the teachings of said applications are to be considered incorporated herein by reference.
It is well known in the glass melting and forming art that forehearths require substantial amounts of heat energy in order to condition the glass received from the furnace temperature to some desired forming temperature at the forehearth outlet. Thus, the forehearth is a net consumer of energy and the anomalous condition exists wherein a large amount of heat energy is required to "cool" the glass to the proper forming temperatures.
It is also known that molybdenum metal (moly), a preferred glass contact material used herein, has significantly higher wear resistance to moving molten glass than conventional refractory materials. However, it is also well known that moly tends to oxidize at temperatures in excess of 550.degree.-600.degree. C., and thus, the moly must be protected from deleterious atmosphere (oxygen) when it is used at or above these elevated temperatures. In Spremulli, '887 for example, the outlet pipe used therein is described as being manufactured from moly components and various protection devices are included therein including cooling means and means for introducing purge gas to protect those portions of the moly pipe that are susceptible to oxidation from the ambience. The Spremulli system, however, requires the use of a conventional forehearth channel for cooling the glass.
The present invention performs the functions of transportation, cooling, and homogenizing molten glass, wherein the useful life of the delivery system is significantly increased and glass-refractory corrosion products, producing glass inhomogeneity, are eliminated. Further, the system greatly reduces the net energy required to condition the glass.